1. Technical Field
The present disclosure relates to power electronic, and more particularly, a power converter and a control method thereof.
2. Description of Related Art
In recent years, with the fast development of the energy conservation technology, more and more customers desire that a switching-mode converter can achieve high conversion efficiency in a wide load range, so improving the efficiency of this converter in a light-load or a no-load is also very important. In this regard, the International Energy Agency (IEA), the United States and Europe and other countries and organizations have been established or are establishing standards to limit a loss of the switching-mode converter in the light-load and the no-load.
By using resonant converter technology, a series resonant DC/DC converter can realize zero voltage switching, and therefore power loss is very small, and power converter efficiency in full-load is high. FIG. 1 is a block diagram of a LLC series resonant DC/DC converter. This topology is commonly based on a frequency modulation mode to stabilize the output voltage by changing the frequency of rectangular waves, in which the duty ratio is 50%, as shown in FIG. 2. The relationship between the power output-voltage gain M and a working frequency is:
  M  =                    2        ⁢        n        *                  V          o                            V        in              =                  1                                                            {                                  1                  +                                                                                    L                        s                                                                    L                        m                                                              ⁡                                          [                                              1                        -                                                                              (                                                                                          f                                s                                                            f                                                        )                                                    2                                                                    ]                                                                      }                            2                        +                                                            Q                  2                                ⁡                                  (                                                            f                                              f                        s                                                              -                                                                  f                        s                                            f                                                        )                                            2                                          .      wherein, the resonance frequency
      f    s    =                    1                  2          ⁢          π          ⁢                                                    C                s                            *                              L                s                                                        .                          ⁢      Q        =                            2          ⁢          π          ⁢                                          ⁢                      f            s                    ⁢                      L            s`                                                8                          π              2                                ⁢                      n            2                    ⁢                      R            L                              .      wherein Ls is a resonant inductance value, Lm is a magnetizing inductance value, Cs is a resonant capacitance value, f is the working frequency of the rectangular wave, n is a turns ratio of a transformer, and RL is a resistance value of the output load.
As shown in FIG. 3, the working method in the light-load is as follows: detecting a state of the load, and controlling the series resonant converter to increase the working frequency of the rectangular wave as the load is decreased; when the working frequency achieves a predetermined value, the working frequency is maintained at this value, and this converter operates in an intermittent control mode (i.e., a hiccup mode).
Existing method for improving the efficiency in the light-load is as follows:                1. Reducing the working frequency of the converter. Because switching losses and driver losses in power devices account for a large proportion of losses in light-load, so reducing the switching frequency can effectively reduce these losses, thus reducing the light-load losses, which applies to PWM (Pulse Width Modulation) circuit.        2. Controlling the converter operates in an intermittent mode. By detecting a voltage error amplifier signal, the converter which is in light-load works in an intermittent mode, so as to reduce the on-off times of the converter switch per unit time, and thereby reducing standby losses.        
In the resonant circuit, only reducing the working frequency cannot effectively control the output voltage, and therefore the above first method cannot be implemented. Above second method may improve the efficiency in the light-load to a certain extent, but when the working frequency of the converter is higher, the gain is less than 1, as shown in FIG. 4. Therefore, in each work cycle, transmission energy is lower; thus the on-off times of the converter switch is still too much, and excessive switching loss and driving loss per unit time are also higher. Thus, the second method cannot reach the limit standard for the light-load loss.
In view of the foregoing, there is an urgent need in the related field to provide a way to improve the efficiency in the light-load more efficiently.